Passive Intermodulation (“PIM”) is a recognized problem in Radio Frequency (“RF”) systems that can result in unintended performance degradation. PIM is a byproduct of RF signals interacting with each other in a system with at least one nonlinear component. The intermodulation interaction between two or more RF signals may result in additional signals formed at the sum and difference frequencies of the original signal frequencies as well as multiples of the sum and difference frequencies. The additional signals can unintentionally appear at frequencies that interfere with intentional signals. For this reason it is desirable to identify and eliminate PIM in most communication systems.
Examples of nonlinearities that may create PIM products include, but are not limited to: loose or misaligned RF connector junctions, metallic flakes or shavings inside of RF connectors or cables, mismatched metallic connections between RF connector surfaces, contaminated surfaces due to dirt, water or oxidation, manufacturing or installation deficiencies damaging RF path components, and nearby objects including metallic guy wires, metal roofs, rusty metal, air conditioner units, etc.
Current systems and methods identify different ways to identify the presence of nonlinearities that may generate PIM interference. A common method describes actively exciting an RF system by two carrier wave (“CW”) signals (i.e., tones). When these tones encounter a nonlinearity, additional intermodulation (“IM”) products will typically be generated in the form of tones at new, different, frequencies. Typically the PIM products of most interest are those related to the original tones by the relationship:
      f    n    =                              n          +          1                2            ⁢              f        1              -                            n          -          1                2            ⁢              f        2            
where:
f1 and f2 are test tones, where f1 is a lower frequency than f2;
fn is the resulting PIM product; and
n is the IM order typically represented as n=±3, ±5, ±7, ±9, etc., for example f3=2*f1−f2 is the frequency of the lower third order PIM product and f3=2*f2−f1 is the upper third order PIM product.
The location of the PIM products are deterministic with the above equation and the presence of a nonlinearity generating the PIM products is typically determined by detecting the existence of at least one said PIM product frequency.
Once the presence of PIM is detected, it is typically desired to find the source and modify it to eliminate the PIM. Finding the PIM source (the nonlinear component) in a RF system is not always a trivial task and typically includes the investment of considerable time, money, and resources. Prior art describes methods of measuring time and phase delays between the transmitted test tones and the received PIM product. These delays can be used to estimate the distance the PIM source is from the test tone transmitter within the RF system path. These methods typically use one receive path to detect the PIM product and are limited by the time and/or phase resolution of the measurement equipment. Highly accurate measurements often require higher cost equipment.
Referring to FIG. 1, a notional drawing for a prior art RF radio tower system and potential PIM source locations is presented. RF radio tower 101 may be supported by guy wires 102 and support antennas 103. Each of the antennas 103 may be connected to a radio transceiver 104 via coaxial cables 105 and coax cable connectors 106. A building 107 may have disposed thereon metallic equipment 108, such as an air conditioning unit or other equipment. As described herein, some or all of these components may act as a source for passive intermodulation distortion. Other metallic objects that are typically in the vicinity of the tower system may include equipment mounting hardware, grounding systems, tower mounting hardware.
Accordingly, there is a need for systems, methods, and/or program products to detect and/or localize a source of passive intermodulation distortion. In an embodiment discussed in further detail below, a method is presented for localizing a source of passive intermodulation distortion, including injecting a signal into a transmit path of an antenna system in a radio frequency system, monitoring a plurality of receive paths of the antenna system, where a first of the receive paths shares an electrical component with the transmit path, and where a second of the receive paths does not share any electrical component with the transmit path, detecting a passive intermodulation distortion product signal on at least one of the plurality of receive paths, and localizing a source of passive intermodulation distortion as being internal or external to the radio frequency system based on the detection of at least one passive intermodulation distortion product signal on the at least one of the plurality of receive paths.
In another embodiment for localizing a source of passive intermodulation distortion, a signal is injected into a first transmit path of an antenna system in a radio frequency system, where the radio frequency system includes a first transmit/receive path comprising the first transmit path and a first receive path which each share a first electrical component, and a second transmit/receive path comprising a second transmit path and a second receive path which each share a second electrical component, and where the first and second transmit/receive paths do not share any electrical components between them, monitoring the first and second receive paths, detecting a passive intermodulation distortion product signal on at least one of the first and second receive paths, and localizing a source of passive intermodulation distortion as being internal or external to the radio frequency system based on the detection of a passive intermodulation distortion product signal on at least one of the first and second receive paths.
In a further embodiment, a non-transitory machine-readable medium is disclosed having stored thereon a plurality of executable instructions to inject a signal into a transmit path of an antenna system in a radio frequency system, monitor a plurality of receive paths of said antenna system, where a first of the receive paths shares an electrical component with the transmit path, and where a second of the receive paths does not share any electrical component with the transmit path, detect a passive intermodulation distortion product signal on at least one of the plurality of receive paths, and localize a source of passive intermodulation distortion as being internal or external to the radio frequency system based on the detection of at least one passive intermodulation distortion product signal on the at least one of the plurality of receive paths.
In yet a further embodiment, A non-transitory machine-readable medium is disclosed having stored thereon a plurality of executable instructions to inject a signal into a first transmit path of an antenna system in a radio frequency system, where the radio frequency system includes a first transmit/receive path comprising the first transmit path and a first receive path which each share a first electrical component, and a second transmit/receive path comprising a second transmit path and a second receive path which each share a second electrical component, and where the first and second transmit/receive paths do not share any electrical components between them, monitor the first and second receive paths, detect a passive intermodulation distortion product signal on at least one of the first and second receive paths, and localize a source of passive intermodulation distortion as being internal or external to the radio frequency system based on the detection of a passive intermodulation distortion product signal on at least one of the first and second receive paths.
Still other embodiments are contemplated wherein systems, circuitry, and/or apparatus are employed to accomplish the localization of a source of passive intermodulation distortion as described herein.